1. Field of the Invention
This invention relates to a glass and a dielectric composition containing the same. It also relates to a multilayer wiring substrate and multilayer ceramic capacitor which are prepared using the dielectric composition.
2. Prior Art
Known substrate materials for forming wiring boards for use in electric and electronic equipment include a recently developed, low-temperature firing material in the form of a mixture of a glass frit having a softening point of about 700.degree. to 900.degree. C. and aluminum oxide. This substrate material can be fired concurrently and integrally with conductors, resistors and the like at relatively low temperatures of, for example, up to 1,000.degree. C.
It was proposed to incorporate dielectric layers having a high dielectric constant in multilayer wiring substrates made of such low-temperature firing substrate materials, thereby building capacitors in the substrates while achieving a size reduction. Typical dielectric layer materials are lead series low-temperature firing dielectric materials (Pb series perovskite compounds) which can be co-fired with the low-temperature firing substrate materials. This system suffers from serious interdiffusion between the dielectric material and the substrate material. One solution is to provide an intermediate layer, but the system still remains unstable and far below the practically acceptable level.
A variety of mixtures of a ceramic filler and glass were proposed as the dielectric composition which can be fired concurrently and integrally with low-temperature firing substrate materials. The glass materials used heretofore are those of lead oxide, silicon dioxide and boron oxide base systems which have several disadvantages including a low dielectric constant, a low glass transition temperature (Tg), and a high average coefficient of linear expansion causing cracks at the interface with the substrate due to thermal strains. For example, Japanese Patent Application Kokai (JP-A) No. 119815/1975 discloses a ceramic composition comprising in admixture polycrystalline ceramic refractory material and interstitial glass. This ceramic composition has a high dielectric constant and is sinterable at relatively low temperatures. The interstitial glass disclosed therein is a lead oxide series glass such as PbO-SiO.sub.2, PbO-Al .sub.2 O.sub.3 -SiO.sub.2 and PbO-ZrO.sub.2 -SiO.sub.2. Such a composition cannot meet all the requirements of dielectric constant, Tg, and an average coefficient of linear expansion.